Gypsum calcium sulfate dihydrate, CaSO4.2H2O comes from a variety of sources. Land plaster is a term for natural gypsum which is any mixture containing more than 50% calcium sulfate dihydrate, CaSO4.2H2O (by weight).
Generally, gypsum-containing products are prepared by forming a mixture of calcined gypsum phase (i.e., calcium sulfate hemihydrate and/or calcium sulfate soluble anhydrite) and water, and, optionally, other components, as desired. The mixture typically is cast into a pre-determined shape or onto the surface of a substrate. The calcined gypsum reacts with the water to form a matrix of crystalline hydrated gypsum, i.e., calcium sulfate dihydrate. It is the desired hydration of calcined gypsum that enables the formation of an interlocking matrix of set gypsum, thereby imparting strength to the gypsum structure in the gypsum-containing product.
Stucco is defined as chemically calcium sulfate hemihydrate and is a well-known building material used to make building plasters and gypsum wallboard. Stucco is typically made by crushing the gypsum rock with and then heating the gypsum at atmospheric pressure to calcine (dehydrate) the calcium sulfate dihydrate into calcium sulfate hemihydrate. In addition to natural gypsum rock the use of Flue Gas Desulphurization gypsum or gypsum from chemical processes can be used as well. Traditionally, the calcining of gypsum has occurred in a large atmospheric pressure kettle containing a mixture of the various phases of the gypsum
U.S. Pat. No. 5,927,968 to Rowland et al., incorporated herein by reference in its entirety, discloses its own method and apparatus for continuous calcining of gypsum in a refractoryless atmospheric kettle. However, U.S. Pat. No. 5,927,968 to Rowland et al. also discloses a variety of kettles for calcining stucco. One such kettle has a thickened dome-shaped bottom, against which a gas-fired flame is directed, with the kettle and burner flame being enclosed in a suitable refractory structure. There is usually an associated hot pit into which the calcined material is fed. The kettle must withstand temperatures in the 2,000-2,400° F. (1093-1314° C.) range. U.S. Pat. No. 5,927,968 to Rowland et al. states U.S. Pat. No. 3,236,509 to Blair typifies this type construction.
U.S. Pat. No. 3,236,509 to Blair, incorporated herein by reference, discloses continuous fluidized kettle calcination in which dried mineral gypsum powder is fed to a covered, but air vented and lightly vacuum exhausted, calcination vessel. After a steady state of operation is attained in the vessel, a substantially continuous stream of cold gypsum that has been pre-dried and ground to a finely divided state and with a wide distribution of fragmented particle sizes, is added on top of the fluidized, boiling mass in the kettle. Under such conditions, the thermal shock upon the cold, dry mineral being dropped into the already boiling mass radically fractures the ground gypsum rock fragments, and the resultant stucco (beta hemihydrate) is highly fractured and fissured, as well as being widely distributed in particle size. This causes the stucco to disperse very rapidly in water, and requires high amounts of gauging water to be mixed with the stucco for rehydration to gypsum at customary use consistencies.
This “dispersed consistency”, also known in the art as “consistency” or “water demand”, is an important property of stucco. Stuccos of lower consistency generally result in stronger casts.
The normal consistency of stucco (gypsum plaster) is a term of art and is determinable according to ASTM Procedure C472, or its substantial equivalents. It is defined as the amount of water in grams per 100 grams of stucco.
U.S. Pat. No. 4,533,528 to Zaskalicky, incorporated herein by reference in its entirety, discloses directly feeding wet chemical gypsum cake to a continuous kettle calciner to produce beta hemihydrate of lower consistency. As explained in Zaskalicky, and also for purposes of the present description, “dispersed consistency” may be defined as the water volume required to give a standard viscosity or flow when a standard amount by weight of stucco is dispersed by mechanical mixing in a laboratory mixer at high shear intensity and for a standard time to equal mixing encountered in the gypsum board forming line, e.g., 7 seconds, or in an industrial plaster formulation casting mixer, e.g. 60 seconds.
For example, as explained in U.S. Pat. No. 4,201,595 to O'Neill, incorporated herein by reference in its entirety, calcined gypsum made by continuous calcination may have a dispersed consistency of about 100-150 cc. “Dispersed consistency” for purposes of gypsum board manufacture may be defined as the water volume required to give a standard viscosity or flow when 100 grams of stucco is dispersed by mechanical mixing in a laboratory high speed blender at high shear intensity and for 7 seconds which is equivalent to the mixing encountered in the board forming line. While the dispersed consistency may be expressed in a particular numerical figure, it will be appreciated that any particular number is variable from one process to the next depending on the particular stucco and the rate of production.
Low consistency stucco is particularly advantageous in automated gypsum board manufacture, in which a large portion of the processing time and processing energy is devoted to removing excess water from the wet board. Considerable excess water is required in gypsum board manufacture to properly fluidize the calcined gypsum and obtain proper flow of the gypsum slurry.
A dispersed consistency value of 100-150 cc. indicates a water requirement of about 85-100 parts of water per 100 parts of the calcined gypsum for a typical slurry in a gypsum wallboard plant. The theoretical water required to convert the calcined gypsum (calcium sulfate hemihydrate or stucco) to set gypsum dihydrate is only 18.7% by weight on a pure basis. This leaves about 67 to about 82% of the water present in the gypsum slurry to be removed in drying the board. Ordinarily, gypsum board dryers in a gypsum board manufacturing line will remove this water, for example, by maintaining the air temperature at about 400° F. (204° C.) and requiring a drying time of about 40 minutes.
U.S. Pat. No. 4,201,595 (also mentioned above), U.S. Pat. Nos. 4,117,070 and 4,153,373 to O'Neill, all incorporated herein by reference in their entirety, teach to lower the dispersed consistencies of continuously calcined kettle stuccos by an after calcination treatment of the stucco with small amounts of water or various aqueous solutions, resulting in a damp but dry appearing material and allowing the small amounts of free water to remain on the calcined gypsum particle surface for a short period of time, about 1-10 minutes for the treated stucco to “heal” .
U.S. Pat. No. 3,410,655 to Ruter et al., incorporated herein by reference in its entirety, teaches producing alpha calcium sulfate hemihydrate. Ruter et al. states the alpha-hemihydrate forms non-needle like crystals, as opposed to the beta calcium sulfate hemihydrate which forms needle-like crystals. Ruter et al. also states the usual plaster of Paris (calcium sulfate hemihydrate) is the beta calcium sulfate hemihydrate. However, depending on the manner of preparation, the plaster of Paris still contains more or less anhydrous calcium sulfate, and/or alpha calcium sulfate hemihydrate. Moreover, plasters with definite alpha-hemihydrate content exhibit higher strengths. Ruter et al. teaches to make alpha calcium sulfate hemihydrate in the form of non-needle-like crystals by elutriating the dihydrate with water to remove organic impurities and fine and slimy crystal portions, forming an aqueous suspension of the dihydrate at a pH about 1.5-6, and subsequently heating under closely controlled conditions.
U.S. Pat. No. 2,907,667 to Johnson, incorporated herein by reference in its entirety, states alpha-hemihydrate is prepared by heating the dihydrate under controlled vapor pressure conditions in the presence of steam or in an aqueous solution.
U.S. Pat. No. 4,234345 to Fässle discloses fast-setting alpha calcium sulfate hemihydrate made from calcium sulfate dihydrate by hydrothermally recrystallizing calcium sulfate dihydrate to form a mixture containing 95%-99% by weight alpha calcium sulfate hemihydrate and 5 to 1% calcium sulfate dihydrate. The dihydrate in this mixture is then converted to beta calcium sulfate hemihydrate by calcining, except for a remainder of up to 0.5 percent of dihydrate, which remains in the mixture.
There is a need for stuccos having low consistency and good strength characteristics.